Transcript Unit 3-2 Nervous System Pt 2 Notes File

Summation
multiple graded potentials add up to reach threshold
•A single EPSP cannot induce an action potential
•EPSPs must summate temporally or spatially to induce an action potential
•Temporal summation – presynaptic neurons transmit impulses in rapid-fire order
•Spatial summation – postsynaptic neuron is stimulated by a large number of terminals at the
same time
•IPSPs can also summate with EPSPs, canceling each other out
Same receptor
activated multiple
times quickly
Different receptors
activated at the
same time
Different receptors
cancel each other
out
What determines if a neuron fires is the total of all the EPSPs (positive) and IPSPs
(negative). If total reaches threshold at the axon, then the neuron will fire (Action
Potential)
Action Potential (nerve impulse)- Phases
1. Resting state – Voltage-gated channels closed
EPSP or a previous action potential triggers 
1. Depolarization Phase – Signaling step
Voltage-gated Na+ channels open (must pass threshold = enough change in membrane
potential)
 Flood of Na+ rushes into cell
 Incoming Na + causes a Large depolarization (inside becomes positive)
 Closes voltage gated Na+-channels,
 Opens voltage-gated K+-channels
 Triggers AP further along the axon
2. Repolarizing phase - END of Action potential signaling
Voltage-gated Na+ channels Close (inactivated / plugged)
Voltage-gated K + channels open
 K + rushes out of cell (exits)
 Outgoing of K + causes repolarization (inside less pos = more neg)
3. Hyperpolarization - period for restablishing resting membrane potential & ion concentrations
i. Voltage-gated Na+ channels resets
ii. Voltage-gated K + channels still open but close eventually
iii. Na+ and K + concentrations are restored by Na+ K + pumps
Propagation - influx of Na+ channels causes depolarization further along membrane
Threshold – Action potential is All-or-Nothing – (point where it is switched on. Like activation
energy) (after that, no going back)
Refractory Period (recovery period)
•Time from the opening of the Na+ activation gates until the closing of inactivation gates (all
gates reset)
•The absolute refractory period:
1. Prevents the neuron from generating an action potential
2. Ensures that each action potential is separate
3. Enforces one-way transmission of nerve impulses
Na+ Voltage gate closed
Inactivation gate open
Na+ Voltage gate closed
Inactivation gate open
(subject to action potential)
However, because of hyperpolarization
(membrane potential is even lower), higher
stimulation required to reach threshold
(subject to action potential)
Both Na+ Gates
opened
(not subject to action
potential)
Na+ Voltage Gate open inactivation
gate closed
(not subject to action potential)
Coding for Stimulus Intensity - Stimulus Strength and AP Frequency
•All action potentials are alike and are independent of stimulus intensity
•Strong stimuli can generate an action potential more often than weaker stimuli
•The CNS determines stimulus intensity by the frequency of impulse transmission
(strength = number of action potentials)
(strength)
Conduction Velocities of Axons
•Conduction velocities vary widely among neurons
•Rate of impulse propagation is determined by:
1. Axon diameter – the larger the diameter, the faster
the impulse
2. Presence of a myelin sheath – myelination
dramatically increases impulse speed
Saltatory Conduction
•Current passes through a myelinated axon only at the nodes of Ranvier
•Voltage-gated Na+ channels are concentrated at these nodes
•Action potentials are triggered only at the nodes and jump from one node to the next
•Much faster than conduction along unmyelinated axons
The Synapse – Transmission of nerve impulse
EPSPs
& IPSPs
build up and activate
Action potential
Synapse
Presynaptic Neuron
Postsynaptic Neuron
•Axodendritic – synapses between the axon of one neuron and the dendrite of another
•Axosomatic – synapses between the axon of one neuron and the soma of another
Other types of synapses include:
•Axoaxonic (axon to axon)
•Dendrodendritic (dendrite to dendrite)
•Dendrosomatic (dendrites to soma)
Soma
Electrical synapses: (very fast)
•less common than chemical synapses
•Correspond to gap junctions found in other cell types
Ion channels are connected between presynaptic and postsynaptic neurons allowing direct
transmission of (ions) electrical signal.
•Are important in the CNS in:
Arousal from sleep
Mental attention
Emotions and memory
Ion and water homeostasis
synapse
Gap junction – a nexus that allows chemical
substances to pass between cells
Chemical Synapses
•Specialized for the release and reception of neurotransmitters
•Typically composed of two parts:
Axonal terminal of the presynaptic neuron, which contains synaptic vesicles
Receptor region on the dendrite(s) or soma of the postsynaptic neuron
Synaptic Cleft
•Fluid-filled space separating the presynaptic and postsynaptic neurons
•Prevents nerve impulses from directly passing from one neuron to the next
•Transmission across the synaptic cleft:
Is a chemical event (as opposed to an electrical one)
Ensures unidirectional communication between neurons
1. Gated Ca2+ opens (activated by action potential)
2. Neurotransmitter is released from presynaptic membrane
• Ca2+ causes vesicles fuse to membrane (exocytosis)
• Neurotransmitters in vesicles released into synaptic cleft
3. Neurotransmitter crosses cleft and binds to receptors in
postsynaptic membrane
4. Neurotransmitter terminated
• Degradation by enzymes
• Reuptake (endocytosis)
• Diffusion away from synapse
Neurotransmitters
•Chemicals used for neuronal communication with the body and the brain
•50 different neurotransmitters have been identified
•Classified chemically and functionally
Chemical Neurotransmitters – Chemical classification
1.
2.
3.
4.
5.
Acetylcholine (ACh) – classic model prev page
•First neurotransmitter identified, and best understood
•Released at the neuromuscular junction
•Synthesized and enclosed in synaptic vesicles
•Degraded by the enzyme acetylcholinesterase (AChE)
•Released by: All neurons that stimulate skeletal muscle
Some neurons in the autonomic nervous system
Biogenic amines
•Include: Catecholamines – dopamine, norepinephrine (NE), and epinephrine
•Indolamines – serotonin and histamine
•Broadly distributed in the brain
•Play roles in emotional behaviors and our biological clock
Amino acids
•Include: GABA – Gamma ()-aminobutyric acid,Glycine, Aspartate, Glutamate
•Found only in the CNS
Peptides -Substance P – mediator of pain signals
Beta endorphin, dynorphin, and enkephalins
•Act as natural opiates; reduce pain perception
•Bind to the same receptors as opiates and morphine
•Gut-brain peptides – somatostatin, and cholecystokinin
Novel messengers: ATP and dissolved gases NO and CO
Functional Classification of Neurotransmitters
•Two classifications: excitatory and inhibitory
Excitatory neurotransmitters cause depolarizations
(e.g., glutamate)
Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine)
•Some neurotransmitters have both excitatory and inhibitory effects
•Determined by the receptor type of the postsynaptic neuron (if receptor causes
depolarization then excitatory, if receptor causes hyperpolarization the inhibitory, you
have to know what the receptor does)
•Example: acetylcholine
•Excitatory at neuromuscular junctions with skeletal muscle
•Inhibitory in cardiac muscle
Postsynaptic Potentials
•Neurotransmitter receptors mediate changes in membrane potential according to:
1. The amount of neurotransmitter released
2. The amount of time the neurotransmitter is bound to receptors
•The two types of postsynaptic potentials are:
1. EPSP – excitatory postsynaptic potentials
2. IPSP – inhibitory postsynaptic potentials
Neurotransmitters and Receptors of ANS
Neurotransmitters
•Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the ANS
•ACh is released by all preganglionic axons and all parasympathetic postganglionic axons
•Cholinergic fibers – ACh-releasing fibers
•Adrenergic fibers – sympathetic postganglionic axons that release NE
•Neurotransmitter effects can be excitatory or inhibitory depending upon the receptor type
Receptors
I - Cholinergic Receptors (receptors that bind ACh) – 2 types
1.Nicotinic
Motor end plates (somatic targets)
All ganglionic neurons of both sympathetic and parasympathetic divisions
The hormone-producing cells of the adrenal medulla
Effect of ACh binding to nicotinic receptors is always stimulatory
2.Muscarinic
Muscarinic receptors occur on all effector cells stimulated by postganglionic cholinergic fibers
The effect of ACh binding:
Can be either inhibitory or excitatory
Depends on the receptor type of the target organ
II - Adrenergic receptors (binds norepinephrine) 2 types - alpha and beta
•Each type has two or three subclasses
(1, 2, 1, 2 , 3)
•Effects of NE binding to:
•  receptors is generally stimulatory
•  receptors is generally inhibitory
•A notable exception – NE binding to  receptors of the heart is stimulatory
Effects of Drugs
•Atropine – blocks parasympathetic effects – blocks ACh binding to receptor
•Neostigmine – inhibits acetylcholinesterase – prolonged stimulation
•Tricyclic antidepressants – prolong the activity of NE on postsynaptic membranes
Blocks reuptake of NE
•Beta-blockers – attach mainly to 1 receptors and reduce heart rate and prevent arrhythmias
•sympathomimetic amine - mimic neurotransmitter
Pseudoephedrine - releases norepinephrine from presynaptic vesicles
NE is in sympathethic axons – stimulates
Electrical synapses: (very fast)
Chemical Synapses – Neurotransmitters
•Direct – Open Ion channels = Channel -inked receptors
•Rapid response (though slower than electrical synapes)
•Examples: ACh and amino acids
i.Composed of integral membrane protein
ii.Mediate direct neurotransmitter action
iii.Action is immediate, brief, simple, and highly localized
iv.Ligand binds the receptor, and ions enter the cells
v.Excitatory receptors depolarize membranes
vi.Inhibitory receptors hyperpolarize membranes
•Indirect - neurotransmitters that act through second messengers (has additional steps before
opening ion channels)
Indirect: G Protein-Linked Receptors
•Responses are indirect, slow, complex, prolonged, and often diffuse
•These receptors are transmembrane protein complexes
•Examples: biogenic amines, peptides, muscarinic ACh receptors, neuropeptides, and those that
bind biogenic amines
Mechanism
1.Neurotransmitter binds to G protein-linked receptor
1.G protein is activated and GTP is hydrolyzed (hydrolysis) to GDP
2.The activated G protein complex activates adenylate cyclase
3.Adenylate cyclase catalyzes the formation of cAMP from ATP
4.cAMP, a second messenger, brings about various cellular responses
•Enzyme activation: some enzymes are non-functional (inactive) until altered by another
molecule and becomes active. ie- by phosphorylation (phosphate group added)
•Some channels are cAMP-gated (ie- Na+ channel that is opened by cAMP)
5.Inactivation – reuptake, degradation
PNS – all neural structures outside the brain and
3 functions of the Nervous System
spinal cord
•Includes sensory receptors, peripheral nerves,
associated ganglia, and motor endings
•Provides links to and from the external
environment
Reflexes
A reflex is a rapid, predictable motor response to a stimulus
Hard-wired (built in) and involuntary (no conscious effort)
Occur in both the Somatic Nervous System and the Autonomic Nervous System
Reflexes may:
•Be inborn (intrinsic) or learned (acquired)
•Involve only peripheral nerves and the spinal cord
•Involve higher brain centers as well
Examples – Blinking eye (somatic), salivation (autonomic)
Reflex Arc
The most basic example of the function of the nervous system
There are five components of a reflex arc
1. Receptor – site of stimulus
2. Sensory neuron – transmits the afferent impulse to the CNS
3. Integration center – either monosynaptic or polysynaptic region within the CNS
4. Motor neuron – conducts efferent impulses from the integration center to an effector
5. Effector – muscle fiber or gland that responds to the efferent impulse
Patellar Reflex Arc –
Somatic Nervous System
•Tapping the patellar tendon stretches the quadriceps and
starts the reflex action
•The quadriceps contract and the antagonistic hamstrings
relax
Functions of the Nervous System
1. Receptors stimulated, activate sensory neuron
2. Processing in the CNS
3. Motor neurons stimulate effector cells
Antagonistic = opposing
5 components of a reflex arc
1. Receptor – Stretching the muscles activates the muscle spindle
2. Sensory neuron – increased rate of action potential
3. Integration center – both monosynaptic or polysynaptic in spinal cord
(very simple, integration for this reflex is only 1 synapse)
4. Motor neuron – increased rate of action potential
5. Effector – quadriceps (muscle fibers) respond to the efferent impulse
Other half of the arc
3b. Integration – polysynaptic, axon collateral stimulates CNS neuron which inhibits motor neuron
(IPSP)
4b. Motor neuron – increased rate of action potential in 2nd motor neuron
5b. Effector – hamstrings relax
Autonomic Nervous System (ANS)
The ANS consists of motor neurons that:
•Innervate smooth and cardiac muscle and glands
•Make adjustments to ensure optimal support for body activities (Homeostasis)
•Operate via subconscious control
•Have viscera as most of their effectors
The ANS differs from the SNS in the following three areas
1.Effectors
•The effectors of the SNS are skeletal muscles
•The effectors of the ANS are cardiac muscle, smooth muscle, and glands
2.Efferent pathways
•Heavily myelinated axons of the somatic motor neurons extend from the CNS to the effector
•Axons of the ANS are a two-neuron chain
i. The preganglionic (first) neuron has a lightly myelinated axon
ii. The ganglionic (second) neuron extends to an effector organ
3.Target organ responses
•All somatic motor neurons release Acetylcholine , and an excitatory effect
•In the ANS:
i. Preganglionic fibers release ACh
ii. Postganglionic fibers release norepinephrine or ACh and the effect is either stimulatory
or inhibitory
iii. ANS effect on the target organ is dependent upon the neurotransmitter released and the
receptor type of the effector
Divisions of the ANS
•ANS divisions: sympathetic and parasympathetic
•The sympathetic mobilizes the body during extreme situations
•The parasympathetic performs maintenance activities and conserves body energy
•The two divisions counterbalance each other (Homeostasis)
Role of the Sympathetic Division (+)
•The sympathetic division is the “fight-or-flight” system
•Involves E activities – exercise, excitement, emergency, and embarrassment
•Promotes adjustments during exercise – blood flow to organs is reduced, flow to muscles is
increased
•Its activity is illustrated by a person who is threatened
Heart rate increases, and breathing is rapid and deep
The skin is cold and sweaty, and the pupils dilate
Role of the Parasympathetic Division (-)
•Concerned with keeping body energy use low
•Involves the D activities – digestion, defecation, and diuresis
•Its activity is illustrated in a person who relaxes after a meal
Blood pressure, heart rate, and respiratory rates are low
Gastrointestinal tract activity is high
The skin is warm and the pupils are constricted
Sensory Receptors
•Structures specialized to respond to stimuli
•Activation of sensory receptors results in depolarizations that trigger impulses to the CNS (Sensory
input)
•The realization of these stimuli, sensation and perception, occur in the brain
Receptor Classification by Stimulus Type
•Mechanoreceptors – respond to touch, pressure, vibration, stretch, and itch
•Thermoreceptors – sensitive to changes in temperature
•Photoreceptors – respond to light energy (e.g., retina)
•Chemoreceptors – respond to chemicals (e.g., smell, taste, changes in blood chemistry)
•Nociceptors – sensitive to pain-causing stimuli
•Osmoreceptors – sensitive to changes in osmotic pressure
•Proprioreceptors – sensitive to relative location of organs
Receptor Class by Location:
Exteroceptors
Respond to stimuli arising outside the body
Found near the body surface
Sensitive to touch, pressure, pain, and temperature
Include the special sense organs
Interoceptors
Respond to stimuli arising within the body
Found in internal viscera and blood vessels
Sensitive to chemical changes, stretch, and temperature changes
Proprioceptors
Respond to degree of stretch of the organs they occupy
Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles
Constantly “advise” the brain of one’s movements
Processing at the Receptor Level
•The receptor must have specificity for the stimulus energy (ie-mechanical force for
mechanoreceptors, light won’t work on mechanoreceptor and tough won’t work on a
photoreceptor)
•The receptor’s receptive field must be stimulated (in range)
•Stimulus energy must be converted into a graded potential
•A generator potential in the associated sensory neuron must reach threshold
(graded potential leads to Action potential)